1. The Technical Field
The present invention relates to a method of determining induced change of polarization state of light in a polarization element, fiber optic sensor devices for determining such induced change of polarization state of light, particularly linear birefringence induced by an electric voltage, an electric field, and a mechanical force, and circular birefringence induced by an electric current and a magnetic field.
A magnetic field, an electric field, or a mechanical force, or a combination of these, can induce anisotropy in the index of refraction of an optically transparent electrooptic, magnetooptic and/or photoelastic material, respectively, which can be detected by a change of polarization state of light.
Linear birefringence can be induced by an electric field in an electrooptic material by the so-called electrooptic Pockels effect and Kerr effect which can be used in the determination of electric voltage and electric field;
circular birefringence can be induced by a magnetic field in a magnetooptic material by the so-called magnetooptic Faraday effect which can be used in the determination of electric current and magnetic field; and PA1 linear birefringence can be induced by a mechanical force in a photoelastic material by the so-called photoelastic effect which can be used in the determination of mechanical force e.g. acceleration and pressure. PA1 a) transmitting unpolarized light from a light source at the one end of an optical transmitter waveguide means to polarizer means at the other end; PA1 b) polarizing the unpolarized light by the polarizer means; PA1 c) transmitting the polarized light through a polarization element; PA1 d) analyzing the transmitted polarized light from the polarization element by an analyzer means; and PA1 e) transmitting the analyzed polarized light from the one end of an optical receiver waveguide means to a light detector at the other end; PA1 a) transmitting unpolarized light from a light source at the one end of an optical waveguide transmitter means to polarizer means at the other end; PA1 b) polarizing the unpolarized light by the polarizer means; PA1 c) transmitting the polarized light through the polarization element using at least one reflective optical element; PA1 d) analyzing the transmitted polarized light from the polarization element by an analyzer means; and PA1 e) transmitting the analyzed polarized light from the one end of an optical waveguide receiver means to a light detector at the other end; PA1 said analyzed light exiting the analyzer means from the same side as the unpolarized light enters the polarizer means; PA1 wherein PA1 f) the unpolarized light while being polarized by the polarizer means, the polarized light while being analyzed by the analyzer means, or both, are non-collimated; and PA1 g) that the light from said other end of the optical transmitter waveguide means in the light path between the polarizer means and the analyzer means is reflected by the at least one reflective optical element focusing the reflected polarized light onto said one end of the optical receiver waveguide means. PA1 i) a sensitive polarization element comprising a suitable polarization active material in which a magnetic field, an electric field, or a mechanical force can induce anisotropy in the index of refraction, and PA1 ii) optionally an auxiliary polarization element providing phase retardation or polarization rotation of polarized light. PA1 a) electrooptic materials including crystals such as CdF, GaAs, GaP, .beta.-ZnS, ZnSe, ZnTe, Bi.sub.4 Ge.sub.3 O.sub.12, Bi.sub.12 GeO.sub.20, Bi.sub.12 SiO.sub.20, KH.sub.2 PO.sub.4, KD.sub.2 PO.sub.4, NH.sub.4 H.sub.2 PO.sub.4, NH.sub.4 D.sub.2 PO.sub.4, LiNbO.sub.3, LiTaO.sub.3, KIO.sub.3, and quartz (SiO.sub.2), and electrooptic polymer manufactured by poling of polymers such as epoxy or polymethylmethacrylate containing an electrooptic dye such as one or more azo dyes such as Disperse Red 1 (4-(4-nitrophenylazo)-N-ethyl-N-2-hydroxyethylaniline), (by poling is meant a process in which the polymer is brought to a phase transition from a mobile state to an immobile state under influence of an electric field, i.e. poling field, such that the alignment to electrooptic dye due to the poling field molecules is preserved after removal of the field. The poling process is described in Allan Gottsche: "Electrooptic and Magnetooptic Sensors for Advanced Applications in Electric Powersystems", Electric Power Engineering Department, Technical University of Denmark Publication no. 9005 (1990)); PA1 b) magnetooptic materials including glasses such as fused silica, and diamagnetic glasses such as BK7, SF6, SF57, SF58, SF59 (Shott), FR-4, FR-5, FR-7 (Hoya), and M-16 (Kigre), and crystals such as quartz (SiO.sub.2,), EuF.sub.2, Tb.sub.3 Al.sub.5 O.sub.12, LiTbF.sub.4, ZnSe, CeF.sub.3, LaF.sub.3, Bi.sub.4 Ge.sub.3 O.sub.12, Bi.sub.12 GeO.sub.20, CdMnTe Y.sub.3 Fe.sub.5 O.sub.12 (YIG) Tb.sub.x Y.sub.x-1 (IG); and PA1 c) photoelastic materials including glasses such as ZKN7, FK5, BK7, SK14, SK16, F2, LaFN2, SF2, SF4, and SF5 (Schott). PA1 a) a polarizer means for polarizing unpolarized light emitted from the one end of an optical transmitter waveguide means transmitting unpolarized light from a light source to the polarizer means; PA1 b ) a polarization element; comprising a suitable polarization active material in which an induced anisotropy in its index of refraction changes the polarization state of said polarized light polarized by the polarizer means; PA1 c) at least one reflective optical element for reflecting said polarized light transmitted through the polarization element; and PA1 d) an analyzer means for analyzing said polarized light reflected by the at least one reflective optical element; said analyzer means positioned at the one end of an optical receiver waveguide means transmitting the analyzed light from the analyzer means to detection means; PA1 said analyzed light exiting the analyzer means from the same side as the unpolarized light enters the polarizer means; PA1 wherein PA1 e) the at least one reflective optical element is arranged to focus light transmitted through the polarizer means from said one end of the optical transmission waveguide means through the analyzer means onto said one end of the optical receiver waveguide means. PA1 i) a convex surface of the polarization element coated with e reflective coating; PA1 ii) planoconvex lens with e convex surface with a reflective coating; PA1 iii) a reflective diffractive optical element; PA1 iv) an optionally coated convex surface of the sensitive polarization element reflecting by total internal reflection; and PA1 v) an optionally coated conves surface of an additional optical element reflecting by total internal reflection. PA1 1) Eugene Hecht: "Optics", 2n Edition, Addison-Wesley Publishing Company Inc., World Student Series (1987); PA1 2) A. Yariv and P. Yeh: "Optical waves in crystals", Wiley-Interscience Publication, (1984); PA1 3) Allan Gottsche: "Electrooptic and Magnetooptic Sensors for Advanced Applications in Electric Powersystems" , Electric Power Engineering Department, Technical University of Denmark Publication no. 9005 (1990).
Generally, induced change of polarization state of light in a polarization element is used in fiber optic polarimetric optical sensors and the induced change can be determined by:
wherein the unpolarized light is collimated by input coupling means between the optical transmitter waveguide means and the polarizer means, and the analyzed polarized light is focused by output coupling means onto the optical transmitter waveguide means, said coupling means consisting of lenses or graded index rod lenses.
Specifically for a fiber optic magnetooptic current sensor, it has been suggested that the transmission of the polarized light through the polarization element is provided by using at least one plane reflective optical surface reflecting a collimated beam.
These optical arrangements have a number of disadvantages.
First of all, a collimated beam or slightly convergent beam is used which requires auxiliary coupling lenses to obtain an optimal light energy transmission from the optical transmitter waveguide means to the optical receiver waveguide means.
Secondly, the use of conventional lenses as coupling means makes the size of the arrangement inconveniently large because the lenses must be surrounded by air in order to obtain a suitably large change of refractive index at the optical surfaces.
Also, mechanical holders for the lenses are required which complicates the mechanical construction.
Further, in the case of a voltage or electric field sensor using conventional lenses, the air present around the lenses limits the electric field that can be applied to the sensor before disruptive electric discharge occurs.
Graded index rod lenses may avoid the problem of air if coupled directly on to the optical waveguide, such as an optical fiber. Such an arrangement can be made compact and the optical interfaces can be filled with an electrically insulating optical cement. However, in order for the graded index rod lenses to collimate the beam from the transmitter, a relative small core diameter (max. 200 .mu.m) optical fiber is required which limits the amount of light that can be launched into the optical fiber by the light source, hereby limiting the signal-to-noise ratio and the sensitivity of such a device, unless compensated for by a more powerful light source.
Thus, cheap visible light emitting diodes e.g. LED's emitting light in the range 500-700 nm cannot be used because they cannot supply sufficient light through such optical fibers having that small core diameter. Instead powerful and more expensive LED's emitting light in the range 800-950 nm must be used. However, LED's and polarizers working at these wavelengths as well as graded index rod lenses are expensive.
Therefore, there is a need for a method of determining induced change of polarization state of light in a polarization element which can be implemented in a compact, small and relative cheap fiber optic sensor device.
2. Prior Art Disclosure
U.S. Pat. No. 3980949 discloses a magneto-optical measuring transducer for measuring both nominal and excessive high currents, said transducer having a first magnetically saturable part and a second paramagnetical part, the outer surfaces of the first and second parts being provided with reflective materials to provide multiple passages of polarized light through the transducer. Nothing is indicated or suggested about providing reflection by means of a reflective imaging optical element and non-collimated light.
W. B. Spillman and D. H. McMahon, "Multimode Fiber Optic Sensors Based on the Photoelastic Effect", Proceedings of Fiber Optics and Laser Sensors, Arlington, Va., Apr. 5-7, 1983, SPIE, Vol.412, Paper No. 412-17, p.110-114 disclose a multimode fiber optic sensor based on the photoelastic effect in which light from an input fiber is collimated by a graded index rod lens, and the analyzed light is separated into two components and injected into two output optical fibers via graded index rod lenses.
UK Patent Application Publication No. 2159944 discloses an optical sensor, in particular an optical pressure sensor, comprising an electrically switchable optical 90.degree. rotator. The sensor uses collimated light and does not comprise a reflective imaging optical element.
U.S. Pat. No. 4613811 discloses a fiber optic magnetooptic current sensor for measuring current flowing through a conductor comprising two components one of which has a plane reflective surface for reflecting a polarized light beam between the two components. Nothing is indicated or suggested about reflecting the polarized light by means of a reflective imaging optical element and non-collimated light.
Hulshof et al., "Optical Voltage Sensor: Application in Electric Power Systems", SPIE Vol. 798, Fiber Optic Sensors II, 1987, disclose a transmission type optical fiber voltage sensor comprising a Pockels cell of bismuth-germaniumoxide and conventional lenses or graded index rod lenses as collimating or focusing coupling means.
EP Patent Application Publication No. 0284346 discloses an optical interface coupled to a Faraday rotator device disposed in surrounding proximity to an energy transmission line in a magnetooptic current transducer which interface comprises a collimator segment coupled to a spacer lens element focusing a divergent beam of input light from a fiber optic cable into a collimated beam directed to the Faraday rotator device through a polarizing element by a parabolic surface.
U.S. Pat. No. 4841234 discloses a fiber optic voltage detector comprising an optical probe having an electro-optic material worked into a frusto-conical shape the tip of which is coated with a reflecting mirror. The optical probe comprises a collimator providing collimated light which is reflected by the reflecting mirror. Nothing is indicated or suggested about providing reflection by means of a reflective imaging optical element and non-collimated light.
U.S. Pat. No. 4948255 discloses a fiber optic optical sensing device comprising an element provided with two total reflection surfaces causing the beam path to invert its direction by 180.degree.. The device uses light collimated by e.g. rod lenses and does not comprise a reflective imaging optical element.